This invention relates to a class of low carbon, martensitic alloys known as maraging steels and to such a maraging steel having a good combination of strength and toughness in the aged condition. The invention relates more specifically to a maraging steel containing relatively low cobalt, having a room temperature yield strength at least as good as known cobalt-containing maraging 250 steels, and also having better room temperature impact toughness than cobalt-free maraging 250 steels.
Maraging steels are a class of low carbon, nickel-iron martensitic steels which achieve high strength levels by the precipitation of intermetallic compounds in an age hardening process. Among the most commonly known maraging steels are the 18 Ni (200), 18 Ni (250), and 18 Ni (300) alloys which have the following typical composition ranges in weight percent.
______________________________________ 18 Ni (200) 18 Ni (250) 18 Ni (300) ______________________________________ Al 0.05-0.15 0.05-0.15 0.05-0.15 Ni 17-19 17-19 18-19 Mo 3-3.5 4.6-5.2 4.6-5.2 Co 8-9 7-8.5 8.5-9.5 Ti 0.15-0.25 0.3-0.5 0.5-0.8 Fe Bal. Bal. Bal. ______________________________________
Included with the balance (Bal.) are the usual amounts of other elements present in commercial grades of maraging steels. Here and throughout this application, percent % will be by weight, unless otherwise stated.
The strength and toughness of the above-described alloys are exemplified by the following typical values for 0.2% Yield Strength (0.2% Y.S.) and Charpy V-Notch Impact Resistance (CVN).
______________________________________ 0.2% Y.S. CVN ksi (MPa) ft-lb (J) ______________________________________ 18 Ni (200) 190-225(1310-1550) 26-50(35-68) 18 Ni (250) 240-265(1655-1825) 18-33(24-45) 18 Ni (300) 260-300(1790-2070) 12-19(16-26) ______________________________________
The above-described alloys are named for their nominal nickel content (18%) and nominal yield strength as indicated by the number in the parentheses. Hereinafter, a particular maraging alloy will be referred to with respect to its nominal yield strength only, unless otherwise noted. For example, 18 Ni (250) will be referred to as a maraging 250 alloy.
Cobalt is a desirable and frequently used element in maraging steels because it helps in providing the high strength levels achieved by such steels. Some maraging steels have at least 7-8 w/o cobalt and some ultrahigh strength grades contain up to 20 w/o cobalt. Cobalt, considered to be a strategic material, is supplied primarily from unstable third world sources. Consequently, cobalt is subject to extreme fluctuations in price and availability.
In response to the supply and cost instabilities of cobalt, so called "cobalt-free" maraging steels containing essentially no cobalt have been developed. U.S. Pat. No. 4,443,254, granted Mar. 22, 1977 to S. Floreen, relates to such a maraging steel containing up to 0.03% C., up to 0.3% Al, 17-19% Ni, 1-4% Mo, 1.25-2.5% Ti, and the balance essentially Fe. The alloy is described as having a yield strength of about 240-250 ksi (1655-1725 MPa), which is comparable to the cobalt-containing maraging 250 alloy. However, this cobalt-free maraging 250 steel leaves something to be desired with respect to toughness since it has comparatively reduced toughness as indicated by a Charpy V-notch impact resistance of only about 10-25 ft-lb (15-34 J).
Furthermore, such cobalt-free maraging 250 steels include higher levels of hardening agents such as titanium. High levels of such hardening agents impart greater strength to the alloy, but also render the material more brittle, more likely to be notch brittle, and more susceptible to stress corrosion cracking.